How relevancy and scoring is done in Elasticsearch?

The Boolean model is used by the Lucene to find the similar documents, and a formula called practical scoring function is used to calculate the relevance.
This formula copies concepts from the inverse document/term-document frequency and the vector space model and adds the modern features like coordination factor, field length normalization as well.
Score (q, d) is the relevance score of document “d” for query “q”.